Tech9
Part of the Furniture
I would assume they are both running the same firmware?
They run on the same firmware branch, but different firmware with different WebUI theme. Both are very similar hardware, but not 1:1 identical. RT model has blue "standard" WebUI theme, GT model has red "gaming" WebUI theme. In terms of user experience - both will be the same. Both are popular around and the usual advice is to get: 1) lower priced model; 2) better looking model as personal preference.
RT-AX88U Pro is the newer device (released after GT-AX6000), some may expect longer support for it, but this is not guaranteed.